High-resistance unit



June 4, 1929. G. A. WELLS 1,715,879

HIGH RESISTANCE UNIT Filed Feb. 3, 1928 ATTORNEY Patented June 4, 1929.

UNITED STATES PATENT OFFICE.

HIGH-RESISTANCE UNIT.

Application filed February 3, 1928. Serial No. 251,541.

This invention relates to resistance units,

particularly to high resistance units used as grid leaks for thermionictubes. An object of my invention is theproduction of a high resistanceunit, the resistance of which varies but slightly over an extendedperiod of time and use.

Another object is the provision of a high resistance unit, which whenused in thermionic tube circuits is substantially noiseless inoperation. 1

A further object is the provision of a high resistance unit which canreadily be calibrated.

Another object is the provision of a simple spring contact arrangementfor gripping a high resistance element firmly and positively therebyassuring good electrical contact.

Still another object .is the provision of an improved method of sealinga high resistance element within an enclosure in a substantiallyair-tight and moisture-proof manner.

Still another object is to provide means of taking up any residualmoisture by use of a getter.

These and other objects and advantages and the manner of attaining themwill appear more clearly in the following discussion.

High resistances of various types, designed particularly for use as gridleaks and resistance coupling units in vacuum tube circuits, are wellknown in the art. For example, resistanccs constructed from india ink,graphite pencil marks, and sputtered-metals, among others, have proveduseful. However, such units, in many cases, having the disadvantages ofproducing noise in the output circuit of vacuum tubes with which theyare connected and of varying in resistance considerably over a period oftime. The noise caused by such grid leaks may be due to a variety ofcauses, such as poor connections to the resistance element and possiblyelectrolytic action in the said element, provided moisture is present,while the change in resistance may be due to many reasons, more or lessobscure. It is difiicult to calibrate grid leak resistances of the typeheretofore known to the mand to produce duplicate units having the samevalue of resistance.

According to my invention, I vproduce a res sistance unit, theresistance of which varies but slightly with age and use, causes but(very little disturbance in the output circuit of the tube with which itis used and can be constructed easily. I accomplish these results byquently be made air-tight, it is prefera larly treated and not show autilizing a body of ceramic or other similar insulating material, whichmay be in the'form of a rod, coated with a mixture of carbon andgraphite that I preferably a ply to the rod' in dry condition. However,best results are obtained by using unglazed porcelain as the body to becoated. When the applied dry there can be little or no electrolyticaction, which action may be caused by the presence of moisture. In orderto insure that the resistance of the finished unit shall remainsubstantially constant, it is preferable thoroughly to clean the rod, toheat it sufiiciently to drive off all moisture, and to keep it in aheated or thoroughly dry atmosphere between the steps of the assemblyprocess. Connections may be made to the coated rod in any suitablemanner but I prefer to utilize spring connections formed by coil springsof such dimensions as to grip the rod tightly when screwed or pushed onit.

y In order to protect the unit from and to insure that the operatingcharacteristics thereof shall not be affected by atmospheric or otherconditions the element may be enclosed in a sealed tube, of glass orother insulating material, provided with suitable contact members at theends. Such an enclosure can be effected by placing the resistance unitwithin a glass tube and then sealing the ends of the tube into metalliccaps by means of solder or other metal such as Woods metal. In sealingthe tube into these caps I find that while solder may be used alone andma freble to utilize with the solder a portion of resin. When solder andresin are used properly a very solid 'and apparently hermetically tightseal is secured. At any rate such a tight seal is obtained that a sealedtube containing powdered fused calcium chloride, or similar hygroscopicor deliquescentmaterial can'be submerged in water for days at a timewithout the calcium chloride or other material showing the slightestevidence of having taken up water, and these leaks may be simichange inresistdamage ance from this cause.-

As the conducting material is applied to the rod in a dry condition, itis Very easy to produce a unit of any desired resistance it beingnecessary merely to apply more material to decrease the resistance or toremove some to increase the resistance. Where a spring contact is usedthe lengthof active rod between the contacts, and hence the resistancemixture is of the unit, can be varied by moving the contacts closertogether or farther apart. This is particularly true of the type ofspring contact which I disclose hereinafter.

In order that the invention shall be understood more clearly referencemay be had to the following description and accompanying drawingsillustrating one embodiment of my invention.

In the drawings, Fig. 1 shows a resistance unit constructed inaccordance with my invention.

Fig.2 shows such a unit in the process of assembly.

In these figures is illustrated a resistance unit comprising a rod 1 ofceramic material such as unglazed porcelain, coated witha mixture ofgraphite and carbon 2 and provided at its two ends with helical springcontact clips 3 each of which may be drawn out to form a connecting wire4 which is positively connectedto the contact caps 5, which may be ofbrass or other metal, by means of solder 6. Positioned around the rod 1and within the caps 6 is a tube of glass or similar material 7. Thistube fits fairly closely within the caps 5 and is held therein by aquantity of solder 6 which is positioned between the outer wall of thetube 7 and the inner wall of the contact cap 5. The connecting wire 4preferably, but not necessarily, extends outwardly into contact with theinner wall of the contact cap 5, and in order to reduce heat conductionfrom the end caps to the coated rod during sealing the wire 4 may beleft or allowed to remain relatively long, or several convolutions ofthe coil spring 3 might be interposed between the rod and wire 4. Theseconvolutions should be slightly spaced from one another in order toreduce heat conduction to the point of contact of spring with the rod.The rod 1 as stated above consists of ceramic material, such as unglazedporcelain. \Vhile I prefer to use a rod of ceramic material such asporcelain, it is possible that there are other materials having suitablecharacteristics, that'might be used as well. The coating 2 on the rodconsists of a mixture of finely divided carbon and graphite, which ismixed in a suitable fashion and applied to the rod in any suitablemanner, for example, as by rubbing it on with the fingers. Theexpression carbonaceous material used in the claims is intended to meansuch materials as carbon or graphite, or the substantial equivalentsthereof, as well as materials capable of reduction to the form ofcarbon, or combinations of such materials. The reason for applying amixture of carbon and graphite to the rod is that sometimes Wheneithercarbon or graphite of certain kinds is used alonethe resistance of thefinished unit will be found to vary with time considerably. Generally inthe ease of carbon alone theresistance increases, while with graphitealone the resistance decreases. I have found that by properly preparinga mixture of carbon black or lamp black and graphite that a coating onthe rod can be made to maintain more nearly a calibrated value ofresistance over a longer period of time and of use than if either isused alone. The propermixture of carbon and graphite will, of course,vary with the particular properties or qualities of the materials used,the proper mixture being determined for different materials byexperimentation. In making the mixture carbon having a positive timeresistance coefficient should be mixed with graphite having a negativetime resistance coet'ticient, in such ratio as to give a resultantmixture having substantially zero coefficient. For example, I have foundthat when carbon produced by Godfrey L. Cabot Inc., of Boston,Massachusetts, under the name Elf-G, and Atcheson graphite are used, theratio of carbon to graphite in the mixture used for coating the rodshould be approximately 1 to 1 or 1 to 1 parts by weight. The carbon andgraphite are placed together ina receptacle and theroughly mixed in anysuitable manner, as by shaking the two materials together when dry. Thismixture is then applied to the rod in dry condition in any desiredmanner. I find that the mixture can be applied satisfactorily by rubbingit on to the rod with the fingers, that is to say, a person can take apinch of the carbon graphite mixture between the fingers and rub itthoroughly on the rod so that the rod acquires a smooth and evencoating. After this has been done suitable contacts are made with theends of the rod, as for example, by means of helices of bronze or otherwire, such" as I have illustrated in the accompanying drawing, and thentheresistance of the element calibrated. If the resistance is found tobe too low, a quantity of the coating material between the contacts canbe rubbed off with a piece of silk cloth or other material. If theresistance is found to be too high more of the mixture can be rubbed on.Instead of the carbon graphite mixture being applied with the fingers itmay be applied by means of brushes, rollers or similar mechanicalequipment, and similarly, in-

stead of rubbing the coating off in order to spring contact members 3which are-slipped over the ends of the rod 1. These contact members 3are preferably applied after the rod has been excessively coated withthe conducting material, and'may be utilized in.

calibrating the resistance of the element by sliding the members closertogether or farther apart, as it is desired to decrease or increase theresistance of the unit. The coil spring member 3 is made of ordinary orphosphor bronze sprin wire and is preferably coiled tightly toget er toa slightly smaller inside diameter than the diameter of the rod to whichit is applied. The ends of the rod might be slig tly chamfered in orderto insure the easy positioning of the spring. The spring is positionedon the coated rod by pushing or screwing in such a manner that the coilsof the spring expand slightly and permit the spring to be pushed ontothe rod. After the spring has been placed on the rod it may be rotatedslightly in the reverse direction in order to insure a more positivecontact and a greater gripping action of the spring on the rod.

In order to protect the element from foreign matter and from mechanicalinjury during use, the tube 7 is provided. As stated above, this tubemay be of glass or any insulating material which will withstand the heattreatment described. This tube is held within the contact caps 5 in anysuitable manner, preferably by means of solder. Woods metal or othersimilar material which is flowed around the outside of the tube betweenthe tube and the inside of the cap, while in a molten condition. Whileplain solder, Woods metal, or

other material alone may be found satisfactory in some cases, a muchmore firm and airtight seal is secured by placing. with the moltensolder a very small quantity of resin. With resin mixed with moltensolder the bond between the glass tube and the cap is so firm that it isimpossible unless heated to remove the cap from the tube withoutbreaking the tube. This joint furthermore is en tirely moisture-proofand appears to be an hermetic one. In making the joint good results canbe secured by using a slow setting solder such as that known as 40-60.

This feature of the glass tube and seal is of considerable importance asby virtue'of it the stability of the operation of the resistance is to alarge extent insured. Various methods may be used for assembling theelement, the contact caps, and tube. For example, I have found that itis possible to make a very satisfactory joint by the following method.First, placing in an empty contact cap a small quantity of solder, withno resin, which is melted by holding the cap in a flame, then thrustinginto the cap the element complete, when within the tube. The metal isthen allowed to solidify, after which the cap is slightly heated in theflame and the glass tube containing the element is worked from the capwhile still having in the end thereof a plug of the solidified metal;which is firmly united to the contact wire of the element and tube also.No resin is used in plugging the tube in the first operation since resinvapor condensing in the element in some cases has a bad effect on theresistance of the element. Next, a piece of hard string 8 is tied by twohalf hitches or a clove hitch around and near the end of the glass tube,as illustrated in Fig. 2, next, a small quantity of solder and resin isplaced in the now empty contact cap and the cap heated to melt thesolder. Next, the glass tube carrying the plug of solder and theresistance unit is then thrust back into the cap. This action forces aportion of the molten solder up around the outside of the glass tubebetween the tube and the inner wall of the contact cap, so as to form incombination with the resin 0. very tight seal. The plug prevents anyfurther quantity of metal from entering the tube. The string placedaround the outside of the tube being pushed further on by the capinsures that the solder and resin shall fill in the entire space betweenthe tube and the cap, as it prevents the molten solder from readilysquirting out of the cap at one place. This process is similar in itseffect to die-casting under pressure. A fter these ste s have been takenthe cap is found to be firmly sealed to the tube and soldered to thecontact wire of the rod. The excess of solder that may have been forcedout past the string can be removed by the thumb nail so as to leave aneat joint. The cap for the opposite end of the unit can be appliedmerely by having within the contact cap a suitable quantity of moltensolder and resin, then after having slipped the same string to andaround the other end of the tube in the manner described above,thrusting slowly into the cap the tube with the first end capped andcontaining the element complete. The molten solder does not rise withinthe glass tube to a very great extent, due to the air pressure withinthe tube, but is squeezed out between the outside wall of the tube andthe inner wall of the cap in the manner described above and therebyforms a perfect seal. Solder alone does not always suffice owing to theunequal expansion of the members, but a mechanically strong joint isobtained by the use of resin in the above described way. Such smallspaces or interstices as may exist evidently are filled by the resin,forming an hermetic seal.

It is preferable before and while carr ing out the steps of the aboveprocess to have all the component parts of the unitthoroughly clean anddry, the rod, the mixture, the glass and contacts all being keptthoroughly heated to drive off all moisture In order to take up anyresidual moisture which may possibly remain inside of the sealed tube, aquantity of fused calcium chloride or phosphorous entoxide may be placedwithin the tube befbre the final sealing operation is carried out to actas a getter.

As the entire make-up of the unit consists of inorganic materials notmixed with any binding material or carrier and protected from moistureand foreign substances, the

operating characteristics of the unit remain constant for a fairly longperiod of time. Furthermore, there is little or no electrolysis or otheraction which would tend to disturb the resistance values of the unit.

In carrying out the above described process I find that ordinarycommercial resin Works well. However, it is possible that other resinsor gums may be found to produce the same result. Vhile I prefer to usesolder and resin as my sealing agent it is possible that other materialssuch as lVoods metal, may be found to act as well with resin as solderdoes.

For making certain types of resistances the mixture of carbon andgraphite set forth in the preceding description may be applied to aceramic rod which has been rubbed with a relatively soft metal so aspartially to coat the rod with particles of the metal. The metalparticles will be spaced from each other slightly and this space will befilled in by the carbon graphite mixture. R'esistances having arelatively low resistance can be constructed in this manner, as well asresistances of higher values. In making such an element copper, lead orthe like can be used to coat the rod. Aside from the coating of therod'with metal particles the construction of a resistance unit would becarried out as described heretofore.

lVhile I have disclosed, for the purpose of illustration, a particularembodiment of my invention, it will be understood that variousmodifications and adaptations, such as would occur to one skilled in theart, may be made without a departure from the spirit of the invention.as set forth in the following claims.

lVhat I claim is:

1. The method of constructing a high resistance unit adapted to operateon low current values which comprises rubbing on an insulating basemember of un glazed porcelain material a quantity of finely dividedcarbon and graphite in a dry thoroughly homogeneous state of admixture.

2. The method of constructing a resistance unit which comprises rubbingon a porcelain rod a mixture of one part Elf-G carbon to one andone-half parts of Atche ou graphite No. 12301. both in a finely dividedand dry state.

3. A resistance element having a small 1 ime resistance. coefficient.comprising a body of insulating material on which has been applied amixture of Elf-G carbon and #2301 Atelieson graphite in a ratio varyingbetween the limits of one to one and one to one and a half parts byweight.

4. A high resistance electrical unit of the (lass described whichcomprises as an essential part of its make-up a base member of unglazedporcelain material substantially impervious to moisture and ordinaryacids, and a uniform substantially homogeneous appropriately attenuatedcoating of dry carbonaceou electrically conducting material carmissesried thereon constituting the electrical resistance element of the unit,thereby providing a high resistance unit which, as compared with unitsof this general class heretofore known, retains a notably constantresistance value for extended periods of continuous operation under the,low current conditions required of apparatus of this class.

5. An electrical resistance unit of high ohmic resistance adapted foroperation on low current values, substantially as described, whichcomprises a base member of unglazed porcelain material and anappropriately thin and uniform dry coating of carbon and graphite inintimate and homogeneous mixture carried thereon constituting the highresistance electrically conducting element of the unit, therebyproviding a high resistance unit capable of continuous operation underthe condition above stated in an atmosphere of reasonably low moisturecontent without substantial change in resistance value for periods oftime notably longer than are obtainable with apparatus of this classheretofore known.

6. An electrical resistance unit of high ohmic resistance adapted foroperation on low current values, substantially as described, whichcomprises a base member of unglazed porcelain material and anappropriately thin and uniform dry coating of carbon and graphite inintimate and homogeneous mixture carried thereon constituting the highresistance conducting element of the unit, a substantiallymoisture-tight enclosure for the said base member and the electricallyconducting coating thereon and a getter within said enclosure for takingup moisture within said enclosure and preventing any substantial actionof moisture upon the said coating or base member, to thereby provide ahigh resistance unit capable of continuous operation under the conditionabove stated under any external conditions of moisture withoutsubstantial change in resistance value for periods of time notably longas compared with apparatus of this general class heretot -re known.

7. The method of making a high resistance electrical unit of the classdescribed which comprises coating a base member of unglazed porcelainmaterial with an electrically conducting carbonaceous material appliedto said base in dry finely divided state in the form of a coatingattenuated appropriately to provide the required high resistance value.whereby a high resistance unit is provided which retains a notablyconstant resistance value for long periods of continuous operation underthe low current conditions required'of apparatus of this class.

8. An electrical resistance unit of high ohmic resistance adapted foroperation on low current values which comprises a base member ofunglazed porcelain material, an

appropriately thin and uniform coating of dry carbonaceous materialcarried thereon constituting the high resistance electrically conductingelement of the unit, and a substantially moisture-tight enclosure forthe said base member and the electrically conducting coating thereon, tothereby provide a. high resistance unit capable of continuous operationunder any conditions of moisture external to the unit withoutsubstantial change in resistance value for periods of time notablylonger than can be attained with apparatus of this general classhereofore known.

9. A high resistanceelectrical unit of the class described whichcomprises as an essential part of its make-up a base member of unglazedporcelain material substantially impervious to moisture and ordinaryacids, and a uniform substantially homogeneous appropriatelyattenuatedcoating of dry carbon carriedthereon constituting theelectrical resistance element of the unit.

10. A high resistance electrical unit of the class described whichcomprises as an essential part of its make-up a base member of unglazedimpervious to moisture and ordinary acids, and a uniform substantiallyhomogeneous appropriately attenuated coating of dry graphite carriedthereon constituting the electrical resistance element of the unit.

In testimony whereof, I have signed my name to this specification this31st day of January, 1928.

GEORGE A. WELLS.

porcelain material substantially

